Poly(ethylene terephthalate), or PET, is one of the most important synthetic fibers as well as is one of the most important engineering plastics. As a synthetic fiber, it can be used separately as long and short fibers or as various types of industrial fibers; it can also be used jointly with cotton or silk fibers in making consumer clothing. As an engineering plastics, poly(ethylene terephthalate) has a wide variety of industrial applications, and can be used in making bottles as well as microwavable kitchen wares. Poly(ethylene terephthalate) also finds many important applications in the food and electronic packing industries.
Poly(ethylene terephthalate) is manufactured commercially by the so-called DMT (dimethyl terephthalate) or TPA (terephthalic acid) process. In the early stages of the development of poly(ethylene terephthalate), because of the inadequate purity of the terephthalic acid raw material, poly(ethylene terephthalate) was manufactured primarily from the DMT process. In the past decades, however, as the purity of terephthalic acid has substantially improved, the TPA process becomes the predominate process for the commercial production of poly(ethylene terephthalate).
In the conventional DMT process for making poly(ethylene terephthalate), dimethyl terephthalate (DMT) and ethylene glycol (EG) are used as the raw materials which are reacted at 220.degree..about.260.degree. C., using at least an acetate of manganese, calcium, or zinc as the catalyst to promote the esterification reaction, which forms an intermediate product bis(2-hydroxyethyl terephthalate) (BHET). The intermediate product, bis(2-hydroxybutyl terephthalate), is then subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 torr) polymerization environment, in the presence of a diantimony trioxide (Sb.sub.2 O.sub.3) or antimony acetate (Sb(OAc.sub.3)) catalyst. After the completion of the reaction, the unreacted ethylene glycol is removed from the reaction product to obtain the poly(ethylene terephthalate) final product.
The TPA process has become the predominate process for the commercial production of poly(ethylene terephthalate). In the conventional TPA process, terephthalic acid (TPA) and ethylene glycol are used as the raw material which are reacted at 220.degree..about.265.degree. C. to effectuate an esterification reaction. The acidic terephthalic itself serves as the catalyst. After condensation (dehydroxylation) as a result of the esterification reaction, an intermediate product of bis(2-hydroxyethyl terephthalate) is formed. The intermediate product of bis(2-hydroxyethyl terephthalate) is similarly subject to a high temperature (250.degree..about.290.degree. C.) and high vacuum (less than 1 torr) polymerization, using diantimony trioxide or antimony acetate as the catalyst. After the reaction, the unreacted butylene glycol is removed from the reaction mixture to obtain the poly(butylene terephthalate) final product.
Currently, the commercial processes in making poly(ethylene terephthalate) also can be classified into two types: batch process and continuous process. In the batch process, the esterification and polymerization reactions are conducted in separate steps--the esterification is proceeded in an esterification vessel, and the intermediate reaction products from the esterification reaction are moved to a polymerization vessel, where the polymerization reaction is carried out. In the continuous process, typically five reaction vessels are used. This compares to the two reaction vessels used in the batch process. In the five reaction vessels used in the continuous process, esterification reaction is carried out in the first two reaction vessels, the third reaction vessel is used for pre-polymerization, and the polymerization reaction is conducted in the last two reaction vessels.
One of the problems that have been encountered in the production of poly(ethylene terephthalate) is the undesired production of side reaction products, which typically appear as having a yellowish color. This problem is more profound with the TPA process. It has been observed that the faster the reaction rate, the greater the amount of the side reaction products that are produced. The presence of the side reaction products also causes the poly(ethylene terephthalate) to be tainted with an undesired and unpleasant yellow color.
Great Britain Pat. No. 1,135,233 teaches a process for the production of fiber-forming polyesters, i.e., poly(ethylene terephthalate), from terephthalic acid in which acetates of manganese or cobalt, or diantimony trioxide is used as a catalyst, and triphenyl phosphate is added as a stabilizer. Japan Laid-Open Pat. App. JP 79-146,893 teaches a catalyst composition containing tetramethyl titanate, cobalt acetate and calcium acetate which improves the yellow color problem observed during the production of poly(ethylene terephthalate) from terephthalic acid. Japan Laid-Open Pat. App. JP 79-163,996 teaches a catalyst composition containing diantimony trioxide, dibutyl tin diacetate, magnesium acetate and calcium acetate which also improves the yellow color problem observed during the production of poly(ethylene terephthalate) from terephthalic acid. Japan Laid-Open Pat. App. JP 80-116,722 teaches a catalyst composition containing tetramethyl titanate, cobalt acetate and calcium acetate which reduces the yellow color problem observed during the production of poly(ethylene terephthalate) from terephthalic acid. Japan Laid-Open Pat. App. JP 80-120,623 teaches the use of a phosphate as stabilizer when a catalyst composition containing diantimony trioxide, cobalt acetate and calcium acetate during the production from terephthalic acid of poly(ethylene terephthalate), which exhibits improved wovability.
Eur. Pat. App. EP 399,742 discloses a catalyst system including, for the first stage, zinc and/or cobalt, and for the second stage, zinc, magnesium, manganese, and/or cobalt, and antimony. Eur. Pat. App. EP 399,799 discloses a direct esterification of terephthalic acid with ethylene glycol followed by the polycondensation of the ester product using a catalyst system containing cobalt, manganese, and antimony components; the prepared poly(ethylene terephthalate) having a predetermined intrinsic viscosity.
U.S. Pat. Nos. 5,008,230, 5,077,259, 5166,311, 5,153,164, and 5,162,488 teach a catalyst system for the production of poly(ethylene terephthalate) from terephthalic acid and ethylene glycol, wherein the catalyst containing antimony, cobalt, and/or zinc, and at least one of zinc, magnesium, manganese or calcium. In these patents, it is taught that, by using the disclosed catalyst system, the reaction rate from terephthalic acid to poly(ethylene terephthalate) can be increased relative to the conventional catalyst, which contains only antimony component (i.e., diantimony trioxide). None of these patents, however, discusses the yellowing problems and the production of side reaction products that are associated with the use of catalyst to accelerate the esterification/polymerization reaction.